Functionally, hematopoietic growth factors can be considered to belong to one of three groups. The first or multilineage group includes interleukin 3 (IL-3) and granulocyte macrophage colony stimulating factor (GM-CSF) which act on early colony forming units (CFU's) including colony forming unit-granulocyte, erythrocyte, megakaryocyte, macrophage (CFU-GEMM), colony forming unit-granulocyte-macrophage (CFU-GM), burst forming units erythrocyte (BFU-E) or megakaryocytes, (BFU-MK). The second or unilineage group includes erythropoietin (EPO), granulocyte colony stimulating factor (G-CSF), interleukin 5 (IL-5), macrophage colony stimulating factor (M-CSF) and thrombopoietin (TPO), and act on later hematopoietic progenitors (i.e., colony forming unit erythrocyte (CFU-E), colony forming unit megakaryocyte (CFU-Mk), and colony forming unit eosinophil (CFU-Eo). The third or “potentiating” group includes interleukin 6 (IL-6), interleukin 11 (IL-11), lymphocyte inhibitory factor (LIF), fibroblast growth factor basic (FGFb), stem cell factor (SCF) and Flt3 ligand (Flt3-L), and act to potentiate the activities of other hematopoietic factors. Within the third group, SCF and Flt3-L both show marked activity on hematopoietic stem cells and thus have been considered special circumstance/stem cell growth factors.
G-CSF and GM-CSF are two commonly used hematopoietic growth factors. The principal action of G-CSF is the stimulation of colony forming unit granulocyte (CFU-G), which in vivo manifests into an augmented production of polymorphonuclear leukocyte (neutrophil) as well as enhancing the phagocytic and cytotoxic functions of neutrophils in general. G-CSF has been shown to be effective in the treatment of severe neutropenia following autologous bone marrow transplantation and high-dose chemotherapy. GM-CSF and G-CSF are each used to decrease the period of neutropenia seen during this type of therapy and thereby reduces morbidity secondary to bacterial and fungal infections. When used as a part of an intensive chemotherapy regimen, G-CSF can decrease the frequency of both hospitalization for febrile neutropenia and interruptions in life-saving chemotherapy protocols. G-CSF also has proven to be effective in the treatment of severe congenital neutropenias. In patients with cyclic neutropenia, G-CSF therapy, while not eliminating the neutropenic cycle, will increase the level of neutrophils and shorten the length of the cycle sufficiently to prevent recurrent infections. G-CSF therapy can improve neutrophil counts in some patients with myelodysplasia or marrow damage. The neutropenia of AIDS patients receiving AZT also can be partially or completely reversed.
G-CSF is typically administered by subcutaneous injection or intravenous infusion at a dose of 1 to 20 μg/kg per day. The distribution and clearance rate from plasma (half-life of 3.5 hours) are similar for both routes of administration. A continuous, 24-hour intravenous infusion can be used to produce a steady-state serum concentration of the growth factor. As with GM-CSF therapy, G-CSF is given daily following bone marrow transplantation or intensive chemotherapy will increase granulocyte production and shorten the period of severe neutropenia. In bone marrow transplantation and intensive chemotherapy patients, continuous daily administration for 14 to 21 days or longer may be necessary to correct the neutropenia. With less intensive chemotherapy, fewer than 7 days of treatment may be needed.
Both G-CSF and GM-CSF will increase the number of marrow progenitor cells in the circulation, a particularly valuable function in patients preparing for stem cell collection. Post-transplant infusions of harvested stem cells together with G-CSF or GM-CSF may reduce the severity of the post-transplant neutropenia.
One hematopaetic growth factor that has recently received considerable attention for its unique properties is Flt3-L. Flt3-L is a transmembrane glycoprotein of approximately 30 kDa. Mouse and human Flt3-L share significant homology at the amino acid level (˜70%), and show cross-species reactivity, so testing human Flt3-L in mouse produces the same or similar biological effects as would occur in the human. Cells known to express Flt3-L include human and mouse T cell lines, as well as architectural cells of the bone marrow, specifically the bone marrow fibroblast.
Some of the myelopoietic, or white blood cell potentiating effects attributed to Flt3-L include: 1) an expansion of CD34+ CD38− cell number when used in conjunction with SCF and IL-3; 2) an increase in high proliferative potential colony forming cells (HPP-CFC) and CFU-GM numbers; and 3) in the presence of GM-CSF, the formation of large numbers of CFU-GM. Individual and direct myelopoietic effects of Flt3-L include an increase in CFU-GM, CFU-GEMM and HPP-CFC survival and a preferential induction of macrophages under certain conditions. Flt3-L alone apparently has minimal or no effects on erythroid and megakaryocyte progenitors.
There is substantial data showing that the system of Flt3-L and its receptor also plays an important role in lymphopoiesis, the processes involved in normal growth and maturation of lymphocytes. This important activity has been confirmed in mice made deficient for Flt3-L System. In these mice hematopoietic populations are essentially normal but marked deficiencies of early B cell progenitors are found in the bone marrow. This has led to the suggestion that Flt3-L, perhaps expressed constitutively by bone marrow fibroblasts, is a normal regulator of B cell lymphopoiesis, while cytokines produced by activated lymphocytes synergize with Flt3-L in times of stress to accelerate B cell development.
In addition to its effects on hematopoietic cells and B cells, Flt3-L has also been shown to stimulate the production of dendritic cells, a highly specialized cell involved in antigen presentation and therefore, normal immunity. Also, with the observation that Flt3-L stimulates the production of dendritic cells, Flt3-L has been identified for potential use in the area of vaccines, both traditional delivery of heat killed or otherwise attenuated agents, as well as protein, peptide or DNA vaccines.
For additional information on Flt3-L, see, for example, Shurin et al., “FLT3: Receptor and Ligand. Biology and Potential Clinical Application”, Cytokine & Growth Factor Reviews, Vol. 9, No. 1, pp. 37-48, 1998.
One of the problems associated with the hematopoietic growth factors such as G-CSF, GM-CSF, SCF and Flt3-L, is the need for multiple daily injections. This, in turn leads to another common disadvantage of current injectable therapies such as these, that being the creation of a saw-toothlike effect of plasma drug levels. This is due to the creation of large bolus bursts of drug shortly after injection, leading to supraphysiologic levels of drug, followed by rapid drops in plasma drug levels as the drug is cleared from the body by normal clearance processes. Upon the next injection, the pattern is repeated with large spikes in plasma levels followed by sub-therapeutic levels until the next injection. An additional problem with current hematopoietic growth factor therapy includes fever and mild-to-moderate bone pain in patients receiving high doses over a long period. In addition, local skin reactions and mild to moderate splenomegaly have been reported.
There is a significant need for improved formulations and methods for delivery of hematopoietic growth factors that address one or more of these problems, especially as treatments involving the use of hematopoietic growth factors continue to expand.